A generic acceleration sensor is described in German Patent No. DE 103 06 707 B4, for instance.
In the context of protecting vehicle occupants in the event of a collision, it is generally conventional to use acceleration sensors to acquire acceleration values at different locations in the vehicle, and to trigger restraints such as airbags and belt tensioners using a control unit as a function of acquired acceleration values. However, the protection system must be monitored for faults, since an undesired triggering of the restraint caused by a malfunction could endanger the occupants of the vehicle as well as other people, especially when the vehicle is moving. Causes of such faulty triggering, e.g., electromagnetic irradiation, thermal stresses or interruptions in the supply voltage of the sensors, may already arise at the level of the acceleration sensors.
To prevent faulty triggering of an airbag already at the level of the acceleration sensor, it is conventional to provide two redundant micromechanical sensor elements (MEMS: MicroElectroMechanical Systems) in the sensor, which mutually plausibilize each other. However, when the signals emitted by the two sensor elements are read into the microcontroller of the control unit with the aid of the jointly utilized A/D converter, which is part of the sensor, faulty triggering may occur despite the redundant sensor elements, if the A/D converter is not functioning properly. To prevent this from happening, an expansion of the redundancy in the acceleration sensor to the entire signal path may occur, so that a separate A/D converter is available for each sensor element. However, since the A/D converters respond very sensitively to their environment or to the operating conditions, such as supply voltage fluctuations, temperature or aging, the parameters jointly used for both A/D converters and relevant for the functionality must additionally be monitored with regard to compliance with specified limits (e.g., relating to low voltages or EMC interference). Straying from such limits could otherwise lead to a common mode signal in both signal paths and thus to a faulty acceleration signal, despite the fact that no acceleration is taking place. The realization of such a redundancy in the supply concept of the A/D converters or the implementation of the required monitoring means or monitors, such as for a low voltage detection, EMC detection, reference voltage and current detection for the A/D converters, is costly and requires large chip surfaces, however.